The present invention relates to a device and to a method for reducing switching losses in power transistors. Every time a power transistor is switched, which acts as a power switch, for example, the charge of the parasitic capacitances of the output stage must be resistively reversed. This results in high power losses.
Conventionally, a step-down converter may be configured with an oscillating circuit made up of a capacitor, which is connected in parallel between the drain and the source of the power transistor, and a coil, which is connected in series to the power transistor. With the aid of the oscillating circuit, the power transistor may be de-energized. The disadvantage here is that the working range of the step-down converter is limited by the serial coil present in the main path of the load current. If the step-down converter leaves the optimal working point, de-energization is no longer possible, and the efficiency of the power transistor decreases considerably.
It is furthermore conventional to connect an auxiliary circuit made up of a power switch, an inductor, and a Schottky diode in parallel to the output stage. A capacitor is connected in parallel to the drain-source terminal of the power transistor of the step-down converter. The power transistor may be de-energized with the aid of the auxiliary circuit. The disadvantage here is that the switching speed is slow due to the parallel capacitance, and the auxiliary circuit requires a higher driver capability. Moreover, only the power transistor of the step-down converter is de-energizable, not, however, the power transistor of the auxiliary circuit, or only within a very limited working range. The power loss of the system is thus not decreased, and both the Schottky diode and the power transistor of the auxiliary circuit have to be dimensioned to be identical to the output stage.
The object of the present invention is to minimize the switching losses of a power transistor.